Area Forecast Discussion
Issued by NWS Boston, MA

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776 FXUS61 KBOX 252010 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 410 PM EDT Fri May 25 2018 .SYNOPSIS...
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High pressure centered south and east of New England will allow for mild temperatures tonight, and another day of warmth for much of Saturday. A cold front drops south across the region Saturday afternoon and evening, bringing scattered showers and a few thunderstorms. This is followed by much cooler and unsettled weather Sunday with some improvement by Monday. Mainly dry and warmer weather returns for Tuesday through Thursday as high pressure moves in.
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&& .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
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4 pm update... The WSW wind gusts of 25-30 kts are expected to diminish with sunset. Surface high pressure well to our SE moves further offshore. Some increase in moisture occurs overnight in prevailing SW flow. While mostly clear skies and dry conditions are expected overnight for most of the area, a few hi-res ensemble members showing possibility of some upstream showers clipping far NE MA after midnight tonight, with some increase in clouds there. Will go with a slight chance pop in that area to account for this possibility. Lows mainly in the upper 50s to low 60s.
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&& .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/...
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Expecting a nearly zonal mid level flow over the USA/Canadian border into early next week, with a few small amplitude waves migrating through the longwave pattern. Of greater significance would be a strong mid level ridge off the SE USA coast near the Bahamas. This ridge should maintain an above normal stream of moisture with tropical origins across the eastern USA. The question for southern New England is whether any northern stream disturbances during the coming week can alter the flow pattern enough to pull this moisture farther north into our area. Model solutions are similar enough to favor a consensus approach through this holiday weekend to smooth over the less predictable details. More significant detail differences for the rest of next week, mainly due to timing issues. Have higher confidence in the forecast through Wednesday, than the latter half of next week. Details... High pressure building over the Maritimes brings increasing east winds and a cool moist marine flow to Southern New England. Gusty winds along the east coast, too. At least a chance of showers Sunday, with a more early spring-like raw feel. Greatest risk for showers toward the eastern half of southern New England. High pressure moves east of the Maritimes on Monday. Surface dewpoints start to slowly rise, and clouds should gradually diminish. Looking mainly dry through Wednesday, as another high pressure moves SE across our region from central Canada. This high pressure should be offshore by Thursday, leading to an increasing risk for showers late Thursday into Friday. Near to above normal temperatures expected Monday through Friday.
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Saturday... There is some uncertainty in exact timing of the back door cold front, which will cross thru the area sometime Sat afternoon and evening, aided by a short wave pushing thru the area. 12Z NAM has slowed down the timing of fropa, holding off until towards 00Z for NE MA. However the 12Z GFS shows the front pushing thru a large portion of southern New England between 18Z and 00Z, and hi-res guidance leans towards the GFS solution. As a result have leaned towards the faster solutions for this forecast package. Temps will tumble behind the front. Anticipate mostly sunny skies in the morning, humid with dew points in the upper 50s to low 60s. Should have enough time prior to fropa for temps to climb into the mid and upper 80s for much of the region, even some highs around 90 possible in the lower CT River Valley. Exception will be across south coastal MA/RI including the Cape/Islands, where SW flow will bring marine influenced air. Then increasing clouds as the back door cold front approaches. Scattered afternoon/evening showers are expected, with a few thunderstorms possible. Low level lapse rates are steep south of the front, but model soundings suggest warm air in the mid levels that hinder the extent of instability. Precipitable water peaks out around 1.25 inches, could see brief locally heavy rainfall. Winds aloft are weak but dry air intrusion in the mid levels could allow for some gusty winds with any thunderstorm. SPC has our area highlighted for general thunder. Saturday night... The cold front progresses thru the remainder of southern New England Sat night, bringing a continued chance for showers and isolated thunderstorms. Wind shift to the N/NE allows cooler marine- influenced airmass to continue pressing for much of the area thru the area, having a noticeably cooler feel. Models show potential for gusts around 20 kts along a portion of the east coastal shoreline. Expect overnight lows for much of the area to dip into the low to mid 50s except possibly parts of SW MA and northern CT where lows in the upper 50s to around 60 are probable, farther away form the ocean influence.
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&& .LONG TERM /SUNDAY THROUGH FRIDAY/...
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Expecting a nearly zonal mid level flow over the USA/Canadian border into early next week, with a few small amplitude waves migrating through the longwave pattern. Of greater significance would be a strong mid level ridge off the SE USA coast near the Bahamas. This ridge should maintain an above normal stream of moisture with tropical origins across the eastern USA. The question for southern New England is whether any northern stream disturbances during the coming week can alter the flow pattern enough to pull this moisture farther north into our area. Model solutions are similar enough to favor a consensus approach through this holiday weekend to smooth over the less predictable details. More significant detail differences for the rest of next week, mainly due to timing issues. Have higher confidence in the forecast through Wednesday, than the latter half of next week. Details... High pressure building over the Maritimes brings increasing east winds and a cool moist marine flow to Southern New England. Gusty winds along the east coast, too. At least a chance of showers Sunday, with a more early spring-like raw feel. Greatest risk for showers toward the eastern half of southern New England. High pressure moves east of the Maritimes on Monday. Surface dewpoints start to slowly rise, and clouds should gradually diminish. Looking mainly dry through Wednesday, as another high pressure moves SE across our region from central Canada. This high pressure should be offshore by Thursday, leading to an increasing risk for showers late Thursday into Friday. Near to above normal temperatures expected Monday through Friday.
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&& .AVIATION /20Z FRIDAY THROUGH WEDNESDAY/...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday Night/...High confidence. Into this evening... VFR all areas. WSW winds with gusts 25-30 kts, diminishing towards/after 00Z. Tonight... SW winds taper but remain brisk over S/SE coast with sustained flow around 10 kts and gusts up to 20 kts. Saturday... SCT SHRA/TSRA activity towards late-day. TEMPO CIG / VSBY impacts with RA/+RA. Can`t rule out brief, gusty winds upwards of 30+ kts with any activity, otherwise NE wind-shift late. Saturday Night...Mainly VFR, with areas IFR possible. Breezy. Chance SHRA, chance TSRA. KBOS Terminal...High confidence in TAF, except moderate confidence in timing of wind shift during Sat. SW winds gusting to 25 kts thru 00Z, can`t rule out some gusts to 30 kts. Back door cold front produces a wind shift from SW to NE around 18Z Sat. KBDL Terminal...High confidence in TAF. SW winds gusting to 25 kts thru 00Z, can`t rule out a few gusts to 30 kts. Back door cold front should hold off until late on Sat/Sat eve which will produce a wind shift from SW to NE. Outlook /Sunday through Wednesday/...Moderate confidence. Sunday: MVFR/IFR conditions possible. Breezy. Chance SHRA with isolated TSRA. Patchy FG. Sunday Night: Mainly IFR, with local MVFR possible. Breezy. Chance SHRA. Patchy BR. Memorial Day: Mainly VFR, with local IFR possible. Monday Night through Wednesday: VFR.
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&& .MARINE...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday Night/...High confidence. Tonight... Small Craft Advisory continues into this evening for all waters, especially S/SE, with increasing westerly winds, gusts around 25 kts, thus allowing waves to build at or above 5 feet. Winds dampening overnight, advisories allowed to drop off except for southern outer coastal waters and RI/BI Sounds, where rough seas continue for the overnight. Saturday... Small Craft Advisory continues into the morning for RI/BI Sounds and into the afternoon for the southern/eastern outer coastal waters. Increasing chances of late day showers and thunderstorms ahead of a NE wind-shift associated with a back door cold front. Gusty winds immediately behind the front may approach 25 kts, which may produce SCA conditions over the northeastern coastal waters. Saturday Night: NE winds (for southern coastal waters SW winds becoming NE during the evening), increasing with gusts to 25 kts possible with seas 2 to 4 feet. SCA headlines may be needed. Chance for showers and isolated thunderstorms. Outlook /Sunday through Wednesday/...Moderate confidence. Sunday: Low risk for Small Craft Advisory winds with local gusts up to 30 kt. Local rough seas. Rain showers likely with a slight chance of thunderstorms. Patchy fog. Areas of visibility 1 to 3 nm. Sunday Night: Winds less than 25 kt. Areas of rough seas. Chance of rain showers. Patchy fog. Visibility 1 to 3 nm. Memorial Day: Winds less than 25 kt. Seas up to 5 ft. Monday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft. Tuesday Night through Wednesday: Winds less than 25 kt.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...Air Quality Alert until 11 PM EDT this evening for RIZ001>008. MARINE...Small Craft Advisory until 8 PM EDT this evening for ANZ230>234-236-251. Small Craft Advisory until 11 AM EDT Saturday for ANZ235-237. Small Craft Advisory until 2 AM EDT Saturday for ANZ250. Small Craft Advisory until 4 PM EDT Saturday for ANZ254>256.
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&& $$ SYNOPSIS...Belk/NMB NEAR TERM...NMB SHORT TERM...NMB LONG TERM...Belk AVIATION...Belk/NMB MARINE...Belk/NMB

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